This invention relates to apparatus for allowing multiple peripheral devices to share a host device. More particularly, this invention relates to an apparatus for switching a host device from a first peripheral device to a second peripheral device upon sensing the presence of the second device. The term "host device" is used herein to described a device to which it is desirable to connect a number of other devices referred to as "peripheral devices." A host device can be a computer to which it is desired to connect peripheral devices such as a modem, printer and plotter. Similarly, a host device can also be a printer to which it is desired to connect peripheral devices in the form of a number of computers.
Computers, calculators, printers and other computer-related devices have a limited number of ports to which other devices may be physically connected. Portable computers such as notebook- or palm-sized devices may have only a single serial port to handle all peripheral devices. With such computing devices a user must repeatedly plug and unplug different peripheral devices as they are required. This effort can prove to be timeconsuming as well as increase the risk of misplacing a peripheral device when it is not plugged in to the computing device's port.
Prior apparatus are known for switching between peripheral devices and have been used for connecting multiple computers to a single printer or multiple printers and plotters to a single computer. These apparatus fall into two broad categories: those that require the user to operate a switch to make a desired connection, and those that scan the multiple ports to look for activity. The apparatus in the first category need not be powered, while those in the second category (known as "smart" switches) require an external, constant source of power, usually from the host device power line.
Both types of switching apparatus have drawbacks. The manually-operable apparatus require user intervention to determine which peripheral device is connected to the host device at a given time. On the other hand, "smart" switching apparatus require constant power, which is not acceptable where the host device is portable and powered by batteries.
The present invention has the advantages of both of these prior types of apparatus without their attendant drawbacks.